Generally, polybutene is prepared by the polymerization of C4 olefins derived during the naphtha cracking under Friedel-Craft type catalyst. The number-average molecular weight (Mn) of polybutene produced in this way is about 300 to 5000. The reactivity of isobutene is highest in olefin components of C4 raffinate-1 so that the produced polybutene is mainly composed by isobutene unit. In the past, polybutene had been used as an adhesive, a glue or an insulating oil so that the product with low reactivity was preferred. However, recently, polybutene to which polar group is introduced is used as an anti-scuff agent, a viscosity index improver etc. or polybutene which is mixed with fuel of an internal combustion engine such as vehicles is used as a refresher.
Most popular material among the products prepared by introducing the polar group into polybutene, is PIBSA (polyisobutenyl succinic anhydrides), which is prepared by reacting polybutene with maleic anhydride. From PIBSA, various lubricant additives and fuel fresheners are prepared. In preparing PIBSA, the nearer to the ends of polybutene a double-bond of polybutene is, the higher a product yield of PIBSA is. On the other hand, in case of the double-bond being inside polybutene, the more the number of alkyl group being substituted on the double-bond is, the lower the reactivity for PIBSA is, owing to steric hindrance thereof, and thus the product yield of PIBSA is reduced. Therefore, studies have been actively performed for preparing high reactive polybutene containing vinylidene of 50% or more, preferably 80% or more.
As a Friedel-Craft type catalyst for preparing high reactive polybutene containing high content vinylidene, boron trifluoride (BF3) is generally used. In polybutene polymerization using a catalyst of boron trifluoride, the reaction product discharged from an exit of the reactor was washed with basic aqueous solution for non-activating and decomposing the catalyst, after the polymerization reaction. The waste water generated at this time contains fluorine components such as HF, boron trifluoride neutralized salt etc. and the concentration of fluorine components in the waste water is very high, for example 5,000˜20,000 ppm, which is varied according to the reaction type. The fluorine is very harmful to aquatic life and human body and therefore the amount of fluorine in the industrial waste water had been strickly regulated.
Most general method for reducing the amount of fluorine contained in the waste water is a Ca-compound addition (Japanese Patent No. 2858478). In this method, Ca compound such as Ca(OH)2, CaCl2 etc. is added to the waste water, and the fluoride is precipitated in the form of insoluble calcium fluoride (CaF2) to remove the fluorine. However, when this method is applied to the waste water containing very high concentration of fluorine, which is produced in the preparing the high reactive polybutene, the remaining amount of fluorine is about 500 to 2,000 ppm and it is not possible to reduce the fluorine content to the desired level. In other way, Al compound having strong affinity for the fluoride is used together with Ca compound, and fluorine component in the waste water can be further removed. However, even in this case, the fluorine content cannot be reduced to the desired level (for example 15 ppm). Since these methods use large amount of Ca compound and Al compound, they have disadvantages of cost for treating waste water being too much and of the amount of sludge, that is precipitation, being increased. The reason why these methods are not effective is that in the waste water exists boron trifluoride neutralized salt which is not removed with the conventional methods. The boron trifluoride neutralized salt is not measured by the general fluorine measurement. But, the salt can be analyzed by adding aqueous sulfuric acid to the waste water and then heating the waste water at 140 to 170° C. so as to decompose. It has been revealed from the analysis that about 10% (500 to 2,000 ppm) of fluorine component contained in the waste water is boron trifluoride neutralized salt. On the other hand, there is another method which uses large amount of diluting water for diluting waste water so as to reduce the amount of fluorine component. However, such a diluting method not only needs additional steps and facilities for diluting but also costs too much in treating waste water and cannot reduce the fluorine discharge amount in the unit process.